This invention relates to boilers.
Boilers which have a pressure shell for containing fluid, such as water or other thermal liquid, are known. Typically, a furnace tube in which fuel is burnt extends at least partly through the shell and is connected at its exhaust end via a smoke or reversal chamber to the first of a plurality of groups or passes of tubes which extend through the shell in heat exchange relationship with the fluid contained therein.
Usually such boilers are operated so that the exhaust gases passed to the stack are still above the dew point of the products of combustion contained in said gases. Consequently, corrosion problems from condensation of acidic combustion products are avoided, and relatively inexpensive materials can be used in the manufacture of the boilers.
The operation of such boilers in that mode means that some of the recoverable heat within the exhaust gases is lost, the heat contained in the exhaust gases being in the form of both sensible heat of the gases and the heat of vaporisation of the condensable combustion products. However, with the recent emphasis on energy conservation, it can be desirable to maximise the recovery of heat even at the expense of higher initial capital outlay.